Bactericide for use in water treatment, method for water treatment and apparatus for water treatment

ABSTRACT

A water-treating microbicide, containing an inorganic acid and a corrosion inhibitor, and further containing a carboxylic acid having 8 or less carbon atoms or any of alkali metal salts thereof. The present invention can provide a water-treating microbicide, water treatment method and water treatment apparatus exhibiting a high sterilization effect in a membrane separation device for seawater desalination, etc.

TECHNICAL FIELD

The present invention relates to a water-treating microbicide, a watertreatment method and a water treatment apparatus.

BACKGROUND ART

Membrane separation techniques are used in wide areas such asdesalination of seawater and brackish water, production of medical andindustrial pure water and ultrapure water, industrial wastewatertreatment and food industry. In such membrane separation, thecontamination of the separation device caused by microbes impairs thequality of the obtained permeating water, and furthermore promotes thegrowth of microbes on the membrane surface and the deposition ofmicrobes and their metabolites on the membrane surface, to lower thepermeability and separability of the membrane. To avoid these seriousproblems, various methods for sterilizing the membrane separation deviceare proposed, and generally, a microbicide is constantly orintermittently added to the feed liquid. As the microbicide, mostgenerally a chlorine-based microbicide advantageous in view of price andoperation is added to achieve a concentration of 0.1 to 50 ppm.Furthermore, an effective sterilization method, in which less expensivesulfuric acid is added to lower the pH of the liquid fed to the membraneseparation device to 4 or less, is also developed (EP1031372A). As thepiping of the membrane separation device, usually a corrosion resistantmetal such as stainless steel is used, but if the addition of sulfuricacid or the like raises the acidity, since the metal goes into thecorrosion region of the Pourbaix diagram, the piping is liable to becorroded. In a state where the acidity is low, there are such problemsthat the sterilization frequency must be increased and that longersterilization time is necessary for enhancing the sterilization effect.

It would therefore be advantageous to overcome the above-mentioneddisadvantages of the prior art by providing a water-treating microbicideand a water treatment method having a high sterilization effect.

SUMMARY OF THE INVENTION

The invention includes the following aspects.

-   A water-treating microbicide, comprising an inorganic acid, a    corrosion inhibitor and a carboxylic acid having 8 or less carbon    atoms or alkali metal salts thereof.-   A water treatment method, comprising the step of adding an inorganic    acid, a corrosion inhibitor and a carboxylic acid having 8 or less    carbon atoms or alkali metal salts thereof to a liquid undergoing    treatment in any steps before a membrane separation step in a water    treatment process using a separation membrane.-   A water treatment method, comprising the steps of adding an    inorganic acid to a liquid undergoing treatment, to keep the pH at 4    or less intermittently, and adding a corrosion inhibitor to the    liquid undergoing treatment, in any steps before a membrane    separation step in a water treatment process using a separation    membrane.-   A water treatment apparatus having a membrane separation device,    comprising a means for adding an aqueous solution containing an    inorganic acid and a corrosion inhibitor to a liquid undergoing    treatment to be fed to the membrane separation device.-   A water treatment apparatus having a membrane separation device,    comprising a means for feeding an aqueous solution containing an    acid and a means for feeding an aqueous solution containing a    corrosion inhibitor, respectively, to the liquid fed to the membrane    separation device.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic view of water treatment apparatus of thisinvention.

The Meanings of selected symbols are as follows:

-   -   1 water intake device    -   2 liquid feed pump    -   3 pre-treatment device    -   4 intermediate layer and safety filter    -   5 booster pump    -   6 membrane separation device    -   7 liquid feed pump    -   8 post-treatment device

DETAILED DESCRIPTION

In the present invention, water treatment refers to a process such asthe desalting, separation or desalination of seawater or brackish water,production of industrial pure water or ultrapure water, industrialwastewater treatment, separation or concentration in the food industry,or recovery of valuable materials from wastewater.

In this invention, a membrane separation device refers to a device, inwhich a liquid undergoing treatment is supplied to a membrane moduleunder pressurization for separation into a permeating liquid and aconcentrate for the purpose of fresh water generation, concentration orseparation, etc. Examples of the membrane module include a reverseosmosis membrane module, ultrafiltration membrane module,microfiltration membrane module, etc. The membrane separation devicescan be classified into a reverse osmosis membrane device,ultrafiltration membrane device and microfiltration membrane device,mainly in reference to the membrane module used.

A reverse osmosis membrane device preferably used in this invention isdescribed below as an example. A reverse osmosis membrane device usuallyconsists of a reverse osmosis membrane element, pressure vessel, boosterpump, etc. The liquid undergoing treatment to be fed to the reverseosmosis membrane device usually contains chemicals such as amicrobicide, coagulant, reducing agent and pH regulator, and it ispretreated by means of coagulation, settling, sand filtration, polishingfiltration, active carbon treatment, microfiltration, ultrafiltration,safety filter permeation, etc., before being fed into the device. Forexample, in the case of seawater desalting, seawater is taken in andseparated from particles, etc. in a settling basin, and a microbicidesuch as chlorine is added to the settling basin for sterilization. Insuccession, a coagulant such as iron chloride or polyaluminum chlorideis added, and sand filtration is carried out. The filtrate is stored ina storage tank and adjusted in pH using sulfuric acid, etc., to be fed.While it is fed, a reducing agent such as sodium hydrogensulfite isadded to reduce and remove the microbicide, and the residue is permeatedsafety filter. Then the filtrate is raised in pressure by ahigh-pressure pump and fed into a reverse osmosis membrane module.However, these pre-treatments are selected as required, depending on theliquid to be treated, application, etc.

The reverse osmosis membrane refers to a semi-permeable membrane thatallows a component in the liquid such as a solvent to permeate but doesnot allow the other components to permeate. The materials generally usedas the reverse osmosis membrane include high molecular materials such ascellulose acetate polymers, polyamides, polyesters, polyimides and vinylpolymers. As for the structure of the membrane, there are, for example,an asymmetric membrane having a dense layer at least on one side of itand having fine pores gradually increasing in diameter from the denselayer into the membrane or toward the other side, and a compositemembrane consisting of the asymmetric film and a very thin active layermade of another material formed on the dense layer. As for the form ofthe reverse osmosis membrane, there are hollow fiber membranes, flatmembrane, etc. Usually it is preferred that the membrane thickness ofhollow fiber membranes or flat membrane is 10 μm to 1 mm, and that theouter diameter of the hollow fiber membranes is 50 μm to 4 mm. As a flatmembrane, an asymmetric membrane is preferred, and as a compositemembrane, it is preferred that a substrate such as a woven fabric,knitted fabric or non-woven fabric, etc. is used as a support. However,the method of the present invention can be used irrespective of thematerial, structure and form of the reverse osmosis membrane, and iseffective in every case.

Typical examples of the reverse osmosis membrane include a celluloseacetate- or polyamide-based asymmetric membrane, and a compositemembrane having a polyamide- or polyurea-based active layer. Among them,a cellulose acetate-based asymmetric membrane and a polyamide-basedcomposite membrane are especially effective for the method of thisinvention, and an aromatic polyamide-based composite membrane is furthereffective.

A reverse osmosis membrane module is a product formed for actually usingthe above-mentioned reverse osmosis membrane. In the case where thereverse osmosis membrane is formed as a flat membrane, it can beinstalled in a spiral, tubular or plate-and-frame module, and in thecase of hollow fiber membranes, they are bundled and installed in amodule. The present invention can be applied irrespective of theseconstitutions of reverse osmosis membranes.

The operation pressure of a reverse osmosis membrane device is usuallyin a range of 0.1 MPa to 15 MPa, and can be selected as required,depending on the liquid to be treated, operation method, etc. In thecase where a solution with a low osmotic pressure such as brackish wateris going to be treated, the device is operated at a relatively lowpressure, and in the case where seawater or industrial wastewater isgoing to be treated, it is operated at a relatively high pressure.

It is preferred that the operation temperature of the reverse osmosismembrane device is in a range of 0° C. to 100° C. If the temperature islower than 0° C., the liquid undergoing treatment may be frozen, and ifhigher than 100° C., the liquid undergoing treatment may evaporate.

The recovery of the liquid undergoing treatment in the reverse osmosismembrane device can be usually selected in a range of 5 to 98%. However,the pre-treatment methods and operation pressure must be taken intoaccount in reference to the properties, concentrations and osmoticpressures of the liquid undergoing treatment and the concentrate, whenthe recovery is set. For example, in the case of seawater desalination,a recovery of 10 to 40% is usually set, and in the case of highlyefficient device, a recovery of 40 to 70% is set. In the case ofbrackish water desalination or production of ultrapure water, operationcan be made usually at a high recovery of 70% or more, as required at 90to 95%. The recovery refers to a value obtained by dividing the amountof the liquid permeating the reverse osmosis membrane by the amount ofthe liquid undergoing treatment, and multiplying the quotient by 100.

A reverse osmosis membrane device mainly consists of a high-pressurepump and a reverse osmosis membrane module. As the high-pressure pump,an optimum pump can be selected in response to the operation pressure ofthe device.

As the reverse osmosis module, one module can be used, but it ispreferred to use plural modules disposed in series or parallel to theliquid undergoing treatment. In the case where they are disposed inseries, a booster pump can be installed between the reverse osmosismembrane modules. In the case of seawater desalination, in view ofequipment cost, especially two modules disposed in series can bepreferably used. In this case, it is preferred to install a booster pumpbetween the reverse osmosis membrane modules disposed in series, forraising the pressure of the liquid undergoing treatment to 1.0˜5.0 MPa,when feeding it to the latter module. If the reverse osmosis membranemodules are disposed in series to the liquid undergoing treatment, theeffect of the present invention is large, since the time during whichthe liquid undergoing treatment is kept in contact with the membranemodules becomes long.

Furthermore, the reverse osmosis membrane modules can also be disposedin series to the permeating liquid. This is a preferable method in thecase where the quality of the permeating liquid is insufficient for useor in the case where it is intended to recover the solute in thepermeating liquid. In the case where the reverse osmosis membranemodules are disposed in series to the permeating liquid, it is preferredto install a pump between the reverse osmosis membrane modules, forre-pressurizing the permeating liquid, or for applying a sufficientpressure in the former step for using the remaining pressure in thelatter step for membrane separation. Furthermore in the case where thereverse osmosis membrane modules are disposed in series to thepermeating liquid, it is preferred to install an acid-adding devicebetween the reverse osmosis membrane modules for sterilizing the latterreverse osmosis membrane module.

In the reverse osmosis membrane device, the portion not permeating themembranes out of the liquid undergoing treatment is taken out as aconcentrate from the reverse osmosis membrane modules. The concentratecan be used or thrown away, or can also be further concentrated by anyother method. The concentrate can also be partially or wholly circulatedinto the liquid undergoing treatment. The permeating liquid that haspermeated the membranes can be used, thrown away or can also bepartially or wholly circulated into the liquid undergoing treatment.

In general, the concentrate of the reverse osmosis membrane device haspressure energy, and for reducing the operation cost, it is preferred torecover the energy. The energy can be recovered by means of an energyrecovery device attached to any desired high-pressure pump, but it ispreferred to recover the energy by a special turbine type energyrecovery pump installed before or after a high-pressure pump or betweenmodules.

It is preferred that the treatment capacity of the membrane separationdevice used in this invention is 0.5 m³ to 1,000,000 m³ as the amount ofwater treated per day.

Furthermore, in the membrane separation device used in this invention,it is preferred that the piping in the device has a structure with fewretaining portions.

In the water treatment method of this invention, an inorganic acid and acorrosion inhibitor are intermittently added to the liquid undergoingtreatment to be fed into the water treatment apparatus. The addition ofan inorganic acid is very important in view of giving the sterilizationeffect, and the effect is remarkable especially in the membranefiltration using seawater as the liquid undergoing treatment. The pH atwhich microbes are killed is peculiar to each microbe species. Forexample, in the case of Escherichia coli, the lower limit of pH forgrowth is 4.6, but the bacterium will be killed at pH 3.4 or less.Seawater contains many kinds of microbes, and they are killedrespectively at different pH values. However, usually if the liquidundergoing treatment is kept at pH 4.0 or less for a certain period oftime, 50 to 100% of microbes can be killed. It is preferred that the pHof the liquid undergoing treatment containing an inorganic acid and acorrosion inhibitor is 3.9 or less. More preferred is 3.7 or less, andespecially preferred is 3.4 or less. There is no particular limit forthe lower limit of pH, but in view of preventing the corrosion ofequipment, 1.5 or more is preferred, and especially 2.0 or more ispreferred.

Furthermore, it is preferred that the pH of the liquid undergoingtreatment is 3.0 or less, for presenting a high sterilization effectagainst microbes including aciduric microbes. If the pH is kept constantat 3.0 or less, a high sterilization effect against all the microbesincluding aciduric microbes can be shown, but the chemical cost formaking the feed liquid acidic becomes high while the effect on thecorrosion of piping equipment threatens to be large. So, it is preferredin view of efficient sterilization, that during ordinary intermittentsterilization, the pH of the liquid undergoing treatment is kept at ashigh as higher than 3.0 in a range of 3.0 to 4.0, and that against themicrobes still remaining to live without being killed, the liquidundergoing treatment is kept at 3.0 or less at a frequency of once per 2to 1,000 times of intermittent sterilization.

It is preferred to intermittently add an inorganic acid and a corrosioninhibitor to the liquid undergoing treatment to ensure that the platecount remaining rate in the concentrate is kept at 30% or less aftercompletion of membrane separation, and that the plate count remainingrate is kept at 15% or less at a frequency of once per 2 to 1,000 timesof intermittent addition of an inorganic acid. If the plate countremaining rate is more than 30%, sterilization is insufficient. Theplate count remaining rate (%) is obtained from the following formula.Plate count remaining rate (%)={(Plate count after adding the inorganicacid)/(Plate count before adding the inorganic acid)}×100

As the inorganic acid used in this invention, any of hydrochloric acid,sulfuric acid, nitric acid, phosphoric acid, etc. can be used, but inview of economic aspect, the use of sulfuric acid is preferred.

The corrosion inhibitor used in this invention is important forpreventing the corrosion of the water treatment apparatus and raisingthe sterilization effect. As the corrosion inhibitor used in thisinvention, a compound selected from polycarboxylic acids having at leastsix carboxylic acid groups in the molecule, ethylenediaminetetraaceticacid, nitrous acid and their alkali metal salts can be preferably used.As the polycarboxylic acid, at least one compound selected frompolyepoxysuccinic acids represented by the following general formula(where n is an integer of 3 or more, and X and Y denote, respectivelyindependently, hydrogen or alkali metal), polyacrylic acid, polymaleicacid, and maleic acid copolymers can be preferably used.

As the corrosion inhibitor, a compound selected from polyepoxysuccinicacids, ethylenediaminetetraacetic acid, polyacrylic acid and theiralkali metal salts is especially preferred. Since these compounds haveatoms with high electro-negativity such as oxygen and nitrogen in themolecule, they are preferably excellent in adsorbability to the surfaceof a metal.

Among them, polyacrylic acid is most preferred since it has high foodsafety and high corrosion-inhibiting effect. Polyacrylic acid isespecially preferred in the case where water treatment is intended forproduction of drinking water.

The optimum range of the weight average molecular weight of polyacrylicacid depends on water treatment conditions such as pH and temperature.So, it is necessary to select polyacrylic acid having a weight averagemolecular weight suitable for the conditions. It is preferred that theweight average molecular weight of polyacrylic acid is in a range of 500to 10,000, and a more preferred range is 1,000 to 8,000. If the weightaverage molecular weight is less than 500, it is difficult to obtain asufficient corrosion-inhibiting effect, and if more than 10,000, thestorage stability of the microbicide is likely to be low.

A polyepoxysuccinic acid or any of alkali metal salts thereof can besynthesized, for example, according to the following method. A maleateis epoxidized using hydrogen peroxide and also using sodium tungstate asa catalyst, to make an epoxysuccinate. Then, the epoxysuccinate ispolymerized with ring opening using calcium hydroxide as a catalyst inan alkali aqueous solution, to obtain a polyepoxysuccinate. As themaleic acid copolymer, a copolymer consisting of maleic acid and anolefin, a copolymer consisting of maleic acid and methyl vinyl ether,etc. can be preferably used.

The acid and the corrosion inhibitor can be added separately to theliquid undergoing treatment to be fed to the water treatment apparatus,or a water-treating microbicide containing both mixed beforehand canalso prepared and added. The preparation of a water-treating microbicidein advance is preferred, since the sterilization treatment can becarried out efficiently.

It is preferred that the concentrations of the inorganic acid and thecorrosion inhibitor in the water-treating microbicide of this inventionare in a range of 50 ppm (weight) to 50 wt % respectively. If theconcentration of each or either of the acid and the corrosion inhibitoris more than 50%, the storage stability of the microbicide is likely todecline. If the concentration of each or either of the acid and thecorrosion inhibitor is less than 50 ppm, it is necessary to increase theadded amount of the water-treating microbicide, and the sterilizationefficiency is likely to decline.

It is preferred that the water used in the water-treating microbicide ofthis invention is pure water. If the water used contains impurities,they may react with the acid or corrosion inhibitor, to form aprecipitate, and the storage stability may decline.

Since the mixture consisting of an acid and a corrosion inhibitor canhappen to be poor in storage stability, it is preferred to further add astorage stabilizer to the water-treating microbicide. As the storagestabilizer, for decreasing the damage to the separation membranes of thewater treatment apparatus and for sustaining the sterilization effect, acarboxylic acid having 8 or less carbon atoms or any of alkali metalsalts thereof can be preferably used. As the carboxylic acid having 8 orless carbon atoms, preferred is at least one selected from acetic acid,lactic acid, succinic acid, tartaric acid, citric acid and malic acid.If such a storage stabilizer is added, the microbicide containing anacid and a corrosion inhibitor can be stored stably for a long period oftime. The optimum range of the concentration of the storage stabilizerin the water-treating microbicide depends on the concentrations of theacid and the corrosion inhibitor in the microbicide, but it is usuallypreferred that the concentration is in a range of 50 ppm (weight) to 50wt %.

The water-treating microbicide of this invention can be used in variouswater treatment processes, but it is preferred to use the microbicide ina water treatment process using a separation membrane greatly affectedby microbes.

The separation membranes that can be used in this invention includereverse osmosis membranes, ultrafiltration membranes, microfiltrationmembranes, etc., but it is preferred that the water-treating microbicideis used in a water treatment process using reverse osmosis membranes forwhich a generally used oxidizing agent such as chlorine cannot be used.

The acid and the corrosion inhibitor can be added separately to theliquid undergoing treatment to be fed to the water treatment apparatus,or a water-treating microbicide containing both mixed beforehand canalso prepared and added. The preparation a water-treating microbicide inadvance is preferred, since the sterilization treatment can be carriedout efficiently.

It is preferred that the water-treating microbicide is added at aconcentration in a range of 10 ppm (weight) to 10 wt % in the liquidundergoing treatment. If the added amount of the microbicide is smallerthan 10 ppm, it is necessary to raise the concentrations of the acid andthe corrosion inhibitor in the microbicide, for obtaining a highsterilization effect, and in this case, the storage stability of thewater-treating microbicide may decline. If the added amount of thewater-treating microbicide is larger than 10 wt %, a large load acts onthe device used for adding the water-treating microbicide, and theenergy consumption becomes large. This may be an economic disadvantage.

It is preferred that the water-treating microbicide is addedintermittently. It is preferred that the period of time during which themicrobicide is kept added each time is in a range of 0.5 to 2.5 hours,and that the addition frequency is once per day to per month. It ispreferred to adequately change the addition period of time and additionfrequency, monitoring the variation in the amount of water permeatingthe membrane, the variations in the plate count and contained organiccarbon of the concentrate, the rise of differential pressure, etc. Forsterilizing the membranes, the membranes can be immersed in an aqueoussolution containing an acid and a corrosion inhibitor while the watertreatment apparatus stops operation, but the method of adding thewater-treating microbicide to the liquid undergoing treatment duringmembrane separation is efficient and preferred.

In the water treatment method of this invention, the inorganic acid andthe corrosion inhibitor can also be added separately to the liquidundergoing treatment. It is preferred that the amount of the inorganicacid added to the liquid undergoing treatment is 10 ppm (weight) or morein view of sterilization effect, and is 1 wt % or less in view ofeconomy and the corrosion prevention of equipment such as piping.

The preferred amount of the microbicide added to the liquid undergoingtreatment depends on the salt concentration of the liquid undergoingtreatment, but it is preferred to control to ensure that the pH of theliquid undergoing treatment becomes 4 or less intermittently, and thatthe concentration of the corrosion inhibitor in the liquid undergoingtreatment is kept in a range of 0.1 ppm to 1%. If the pH of the liquidundergoing treatment becomes higher than 4, the sterilization effect candecline. Furthermore, if the concentration of the corrosion inhibitor islower than 0.1 ppm, the corrosion-inhibiting effect may decline. On thecontrary, if the concentration of the corrosion inhibitor is higher than1%, the corrosion-preventing effect tends to level off, and this may bean economical disadvantage.

If sulfuric acid is used as the inorganic acid, it is preferred to keepthe added amount proportional to the salt concentration of the liquidundergoing treatment. For example, when 50 ppm of sulfuric acid wasadded to a pressure-sterilized (120° C., 15 minutes) physiological saltsolution (salt concentration 0.9 wt %), the pH dropped to 3.2, but whenseawater samples collected at three places and a commercially availableartificial seawater sample (salt concentration about 3.5 wt %) werepressure-sterilized (120° C., 15 minutes) and used as liquids undergoingtreatment, the pHs of the liquids undergoing treatment were in a rangeof 5.0 to 5.8 even if 100 ppm of sulfuric acid was added. This isconsidered to be mainly the effect due to the M alkalinity of seawater.To keep the pH of seawater at 4 or less, it is usually preferred to add120 ppm (weight) or more of sulfuric acid. It is preferred that theupper limit in the added amount of sulfuric acid is 400 ppm or less inview of economy and the corrosion prevention of equipment such aspiping. More preferred is 300 ppm or less. When the concentrations ofsulfuric acid added to the above natural seawater and artificialseawater samples were 150 ppm and 200 ppm, the pH values of the liquidsundergoing treatment were respectively 3.2 to 3.6 and 2.8 to 2.9. Thatis, with the increase in the concentration of sulfuric acid, the pHvariation of the liquid undergoing treatment decreases.

The optimum range of the concentration of the corrosion inhibitor in theliquid undergoing treatment depends on the liquid to be treated andwater treatment conditions, but a range of 0.1 ppm (weight) to 1 wt % isusually preferred. In view of economy and the convenience of watertreatment operation, a range of 1 to 500 ppm is more preferred. Forexample, in the case where wastewater with a pH of 1.0 and a saltconcentration of about 8% is treated at a temperature of 35° C., it ispreferred that the concentration of the corrosion inhibitor is in arange of 1 to 100 ppm in the liquid undergoing treatment.

In this invention, the inorganic acid and the corrosion inhibitor can beadded at any step before the liquid undergoing treatment is fed to themembrane separation device. For sterilization of the membrane separationdevice, it is preferred to add them immediately before the membraneseparation device. Furthermore, it is preferred to add the inorganicacid on the downstream side of adding the corrosion inhibitor to theliquid undergoing treatment, for inhibiting the corrosion of piping.

It is also a preferred method to add the corrosion inhibitorsimultaneously when the inorganic acid is added. In the case where thecorrosion inhibitor is expensive, it is preferred, in view of economy,to add it only when the pH of the liquid undergoing treatment is 3.0 orless.

It is preferred to intermittently add the inorganic acid and thecorrosion inhibitor. It is preferred that the period of time duringwhich the inorganic acid and the corrosion inhibitor are kept added eachtime is in a range of 0.5 to 2.5 hours, and that the addition frequencyis once per day to per month. It is preferred to adequately change theaddition period of time and addition frequency, monitoring the variationin the amount of water permeating the membranes, the variations in theplate count and contained organic carbon of the concentrate, the rise ofdifferential pressure, etc. For sterilizing the membranes, the membranescan be immersed in an aqueous solution containing the acid and thecorrosion inhibitor while the water treatment apparatus stops operation.

In the case where the inorganic acid and the corrosion inhibitor areadded separately, the addition frequency of the inorganic acid can bedifferent from that of the corrosion inhibitor. For example, the acidcan be added for 0.5 to 2.5 hours every other day, and the corrosioninhibitor can be added for the same period of time but at a differentfrequency, say, once per week. Especially if the corrosion inhibitor isexpensive and excellent in the corrosion-inhibiting effect, it ispreferred to lower the addition frequency of the corrosion inhibitor inview of economy, for example, to combine the addition of the acid onlyand the addition of both the acid and the corrosion inhibitor.

The water treatment apparatus having a membrane separation device of thepresent invention consists of, for example, the following A to H.

-   A. Water intake device: This device takes in the liquid undergoing    treatment as the raw water, and usually consists of an intake pump,    chemical injection equipment, etc.-   B. Pre-treatment devices communicating to the intake device: These    devices pre-treat the liquid undergoing treatment to be fed to the    membrane separation device, for removing the suspended matter,    emulsified product, etc. in the liquid undergoing treatment, and    inject some chemicals.    For example, the devices are disposed in the following order.-   B-1 Coagulation filtration device-   B-2 Polishing filter    An ultrafiltration device and a microfiltration device can also be    used instead of B-1 and B-2.-   B-3 Chemical injecting equipment for injecting a coagulant,    microbicide, pH regulator, etc.-   C. Intermediate tank installed as required to communicate to the    pre-treatment devices: Having such functions as water quantity    control and water quality buffer action.-   D. Filter communicating to the intermediate tank, if it is    installed, or to the pre-treatment devices, if the intermediate tank    is not installed: Having a function of removing solid impurities of    the liquid undergoing treatment to be fed to the membrane separation    device.-   E. Membrane separation device: Consisting of a high-pressure pump    and a membrane separation module.

Plural membrane separation devices can also be installed in parallel orin series. If they are installed in series, a pump can be installedbetween the membrane separation devices for raising the pressure of theliquid undergoing treatment to be fed to the latter membrane separationdevice.

-   F. Post-treatment devices communicating to the permeating water    outlet of the membrane separation device. For example, the following    devices can be exemplified.-   F-1 Degasifier: Having a function of removing carbonic acid.-   F-2 Calcium column-   F-3 Chlorine-injecting device-   G. Post-treatment devices communicating to the outlet on the raw    water side of the membrane separation device. For example, the    following devices can be exemplified.-   G-1 Buffer: For example, a neutralizer.-   G-2 Discharge equipment-   H. Others

A wastewater treatment device, etc. can also be installed as required.

The water treatment apparatus of this invention can have a pumpinstalled at a desired place. Furthermore, it is preferred that one ormore means for adding the inorganic acid and the corrosion inhibitor ortheir aqueous solutions are installed in the intake device A or thepre-treatment devices B or before the pre-treatment devices B, or beforeor after the filter D. Especially it is preferred to install the meansbefore the membrane separation device, i.e., before or after the filterD.

To enhance the effect of this invention, it is preferred that thedevices used for adding the water-treating microbicide, inorganic acidand corrosion inhibitor can be automatically controlled, and arerespectively provided with a pump capable of adequately controlling theinjected amount. Furthermore, it is preferred to install the instrumentsfor measuring the pH values of the liquid undergoing treatment to be fedand the concentrate, the concentration of the corrosion inhibitor, etc.in the apparatus. Moreover, to control the intermittent addition ofwater-treating microbicide, etc., it is preferred that an instrumentcapable of measuring time is provided. It is more preferred that anautomatic controller allowing the automatic operation of the watertreatment apparatus as a whole is provided.

It is preferred that the components of the water treatment apparatus ofthis invention such as piping and valves are made of materials unlikelyto be corroded at pH 4 or less. If the liquid undergoing treatment to befed is kept at pH 4 or less, a high sterilization effect can beobtained, and the effect of removing the scale in the piping can also beobtained. To prevent the membrane deterioration caused by oxidizingagents of chlorine, etc., sodium hydrogensulfite is added as the casemay be, but, since the water-treating microbicide of the presentinvention is used, the amount of sodium hydrogensulfite can beremarkably decreased.

The addition of a chlorine-based microbicide in a pre-treatment step iseffective for sterilization and is generally used. In the case of atreatment apparatus having a membrane separation device, achlorine-based microbicide is continuously or intermittently injected,for example, in any step of the devices A to D. This method can almostperfectly sterilize the liquid undergoing treatment to be fed unless aresistant strain emerges. A chlorine-based microbicide can chemicallydeteriorate a reverse osmosis membrane. To prevent deterioration, areducing agent such as sodium hydrogensulfite is generally addedimmediately before the membrane separation device. However, in theliquid undergoing treatment remaining after reducing and removingchlorine by a reducing agent, microbes can easily grow. In addition, themicrobes are not a variety of microbes existing in the raw seawaterbefore the addition of the microbicide, but a group of as sortedmicrobes that may include many aciduric microbes. This problem can besolved if the addition of a chlorine-based microbicide in thepre-treatment step and the injection of a reducing agent immediatelybefore the membrane separation device are carried out respectivelyintermittently. This method is also effective for preventing membranedeterioration. It is preferred that the chlorine-based microbicide isinjected once per day to once per six months for about 30 minutes to 2hours each time, in reference to the quality of raw seawater, i.e., theexistence of microbes. In adaptation to the timing of adding thechlorine-based microbicide, and considering the movement of the watercontaining the chlorine-based microbicide, it is preferred to supply areducing agent at a position between the pre-treatment devices and themembrane separation device for inactivating the chlorine-basedmicrobicide. In addition, in adaptation to the timing, it is desirableto add the water-treating microbicide of this invention or to add thecorrosion inhibitor and the acid separately to the aqueous solution tobe fed to the membrane separation device, for sterilizing the membraneseparation device.

The intermittent chlorine-based microbicide injection method to thepre-treatment step like this gives an effect of remarkably decreasingthe treatment cost such as microbicide cost compared with the continuousinjection of microbicide. This can be achieved for the first time withthe water treatment method of the present invention using thewater-treating microbicide or the acid and the corrosion inhibitor, andcould never be achieved by the conventional sterilization methods sincethe sterilization effect is insufficient.

The water treatment method and apparatus of this invention can besuitably used for the water treatment with a membrane separation device.Particularly it can be suitably used for water refining processes suchas desalination of seawater, desalination of brackish water, productionof industrial water, production of ultrapure water or pure water,production of medicinal pure water, clarification of raw tap water, andadvanced treatment of tap water. Furthermore, it can be used in theconcentration of food, or in the case where organic materials, etc.likely to be decomposed by conventional oxidizing microbicides areseparated or concentrated, so that they can be concentrated or recoveredwithout being decomposed. Thus, the effect of this invention is large.Moreover, in the case of producing drinking water, this invention has aneffect that the generation of the trihalomethane produced with chlorinesterilization can be prevented. Still furthermore, the water treatmentmethod of this invention is especially suitable for the production ofdrinking water, since compounds with high food safety only can be usedfor sterilization.

EXAMPLES

This invention is described particularly in reference to examples, butis not limited thereto or thereby.

First of all, the synthesis of the chemical solution used in theexamples is described below.

<Example of Synthesizing a Polyepoxysuccinate>

An epoxysuccinate was synthesized as described below according to themethod of Payne, et al. (J. Org. Chem., 24, 54 (1959)).

A 2-liter three-neck flask was charged with 280 g of maleic anhydrideand 428 ml of ultrapure water for dissolution. To the aqueous solution,500 g of 48 wt % potassium hydroxide aqueous solution was added dropwiseusing a dropping funnel with cooling to keep the temperature at roomtemperature. Then, 18.8 g of sodium tungstate was added, andsubsequently 332 g of 35% hydrogen peroxide water was added dropwise.The mixture was stirred for about 30 minutes, and 115 g of 48 wt %potassium hydroxide aqueous solution was gradually added. In this case,the flask was quickly cooled to keep the reaction temperature at 55 to65° C. Then, the reaction mixture was kept at 65 to 60° C. for 30minutes, to obtain a potassium epoxysuccinate aqueous solution. Theaqueous solution was cooled to room temperature and concentrated to 300ml, and it was poured into 1 liter of acetone. The produced precipitatewas secured by filtration, for isolation as potassium epoxysuccinate.

Then, a 200 ml round bottom flask was charged with 10.4 g of thepotassium epoxysuccinate and 50 g of ultrapure water, and 48 wt %potassium hydroxide was added, to adjust the pH of the aqueous solutionto 10.3. Furthermore, 0.41 g of calcium hydroxide was added, andreaction was carried out at 80° C. for 6 hours. In succession, thereaction mixture was cooled to room temperature, and the insolublematter was secured by filtration. A rotary evaporator was used to removewater at a bath temperature of 40° C., to obtain a white solid.

The molecular weight of the obtained polyepoxysuccinate was measured bymeans of gel permeation chromatography (GPC). Concretely a sample wasprepared at a concentration of 200 ppm, and as a standard substance,polyethylene glycol with a known molecular weight was used, to draw acalibration curve, for calculating the molecular weight of the sample.The weight average molecular weight of the obtained polyepoxysuccinateacid was Mw=20900 (n=100, Mw/Mn=1.00).

Examples 1 to 3

Twenty weight percent of sulfuric acid and 0.1 wt % of a corrosioninhibitor shown in Table 1 were added to pure water (electricconductivity 10 μS/cm), to prepare a water-treating microbicide (pH0.6). Stainless steel test pieces (20 mm×30 mm×1 mm) made of SUS316L andpolished with a No. 320 file on the surface were washed with pure waterfor 60 minutes using an ultrasonic washer, washed with acetone for 60minutes, and dried in air. The water-treating microbicide was dilutedwith seawater (electric conductivity 100 mS/cm) to 100 times(microbicide concentration 1 wt %), to make 100 ml of a testing liquid(pH 1.2), and it was placed in each of ten 100 ml polyethylenecontainers. The stainless steel test pieces were immersed in thecontainers one by one. The containers were allowed to stand in an 80° C.thermostatic chamber. On the day 4^(th) day and 7^(th) day after startof immersion, the test pieces were taken out and weighed. The testpieces were washed with pure water for 5 seconds, washed with acetonefor 5 seconds, dried in air, and weighed in a silica gel-driedatmosphere using an electronic balance capable of weighing in 0.01 mg.The average value of five test pieces was obtained. The effect of addingthe corrosion inhibitor was evaluated as described below.

The weight loss in the period from start of immersion to the 4^(th) day(a) and the weight loss in the period from the 4^(th) day to 7^(th) day(b) were obtained respectively as follows.Weight loss (a) (g/m²)=(Weight of test piece before immersion−weight oftest piece on the 4^(th) day)/Surface area of test pieceWeight loss (b) (g/m²)=(Weight of test piece on the 4^(th) day−Weight oftest piece on the 7^(th) day)/Surface area of test piece

Then, the ratios of the weight losses (a) and (b) caused with the use ofcorrosion inhibitor to the weight losses (a) and (b) caused without theuse of corrosion inhibitor were obtained respectively as describedbelow.Weight loss (a) ratio=Weight loss (a) caused with use of corrosioninhibitor/Weight loss (a) caused without use of corrosion inhibitorWeight loss (b) ratio=Weight loss (b) caused with use of corrosioninhibitor/Weight loss (b) caused without use of corrosion inhibitor

The average value of the weight loss (a) ratio and the weight loss (b)ratio was employed as the weight loss rate. The result is shown inTable 1. (In the table, potassium polyepoxysuccinate is abbreviated asPES, tetrasodium ethylenediaminetetraacetate, as EDTA, and polyacrylicacid, as PA.)

Comparative Example 1

An experiment was carried out as described for Example 1, except that nocorrosion inhibitor was added. The result is shown in Table 1. In thiscomparative example without using any corrosion inhibitor, the weightloss rate was larger than those in Examples 1 to 3, to show that thetest pieces were corroded heavily.

TABLE 1 Corrosion inhibitor Weight loss rate Example 1 PES 0.37 Example2 EDTA 0.58 Example 3 PA 0.67 Comparative Nil 1.00 Example 1

Examples 4 to 7

Sulfuric acid was added to seawater (electric conductivity 100 mS/cm)for adjusting its pH to 1, and 10 ppm of a corrosion inhibitor shown inTable 2 was added to the seawater, to prepare a testing liquid. Testpieces (20 mm×30 mm×1 mm) made of SUS304 and polished with a No. 320file on the surface were washed with pure water for 60 minutes using anultrasonic washer, washed with acetone for 60 minutes, and dried in air.One hundred milliliters of the testing liquid was placed in each of five100 ml polyethylene containers, and the stainless steel test pieces wereimmersed in the containers one by one. The polyethylene containers wereallowed to stand in a 35° C. thermostatic chamber for 3 days, heated to80° C. and allowed to stand continuously for 17 hours. The test pieceswere taken out, washed with pure water for 30 seconds and washed withacetone for 10 seconds. The weight loss due to corrosion was measured asdescribed below. The weight loss was obtained from the followingformula, and the average value of five samples was adopted:Weight loss (g/m²)=(Weight of test piece before immersion−Weight of testpiece after 3 days of immersion)/Surface area of test piece

The result is shown in Table 2. (In the table, potassiumpolyepoxysuccinate is abbreviated as PES, tetrasodiumethylenediaminetetraacetate, as EDTA, and butanetetracarboxylic acid, asBTC.)

Comparative Example 2

An experiment was carried out as described for Example 4, except that nocorrosion inhibitor was used.

As can be seen from Table 2, at a strong acid condition of pH 1, in thecase where a corrosion inhibitor was added, a high corrosion-inhibitingeffect was shown compared with the case where no corrosion inhibitor wasadded.

TABLE 2 Concentration Weight loss Corrosion inhibitor (ppm) (g/m²)Example 4 PES 10 0.19 Example 5 EDTA 10 0.14 Example 6 PA 10 0.38Example 7 BTC 10 17.9 Comparative Nil — 27.9 Example 2

Example 8 and Comparative Examples 3 and 4

Twenty weight percent of sulfuric acid and 0.1 wt % of a corrosioninhibitor shown in Table 3 were added to pure water (electricconductivity 10 μS/cm), to prepare a microbicide for a water treatmentapparatus. In Example 8, 0.5 wt % each of sodium citrate and malic acidwere further added as storage stabilizers. Stainless steel test pieces(20 mm×30 mm×1 mm) made of SUS304 and polished with a No. 320 file onthe surfaces were washed with pure water for 60 minutes using anultrasonic washer, washed with acetone for 60 minutes, and dried in air.Then, the stainless steel test pieces were immersed in 50° C. 20% nitricacid water for passivation treatment for 1 hour, taken out, washed withacetone and dried in air. The microbicide was diluted with seawater (100mS/cm) to 100 times (microbicide concentration 1 wt %), to make atesting liquid (pH 1.4), and the testing liquid was placed in five 100ml polyethylene containers. The stainless steel test pieces wereimmersed in the containers one by one. The polyethylene containers wereallowed to stand in an 80° C. thermostatic chamber. On the 6^(th) dayafter start of immersion, the test pieces were taken out and weighed.The test pieces were washed with pure water for 5 seconds, washed withacetone for 5 seconds, dried in air, and weighed in a silica gel-driedatmosphere using an electronic balance capable of weighing in 0.01 mg.The weight loss was calculated as described for Example 4. The result isshown in Table 3.

Separately seawater with a salt concentration of 6.9 wt % was allowed tostand at 30° C. overnight to stabilize the plate count, and diluted withsterile water to 3.5 wt % (this is called solution A). To seawater(electric conductivity 100 mS/cm), 0.1 wt % of a microbicide shown inTable 3 was added (pH 3.1) and the solution was allowed to stand at 30°C. for 30 minutes (this is called solution B). For obtaining the platecount, a medium for marine bacteria was used to culture at 30° C. for 6days, and the number of emerging colonies was counted. The plate countremaining rate to the plate count obtained without pH regulation(solution A) was obtained. That is, the plate count remaining rate wasobtained from the following formula.Plate count remaining rate (%)=[{Plate count after reaction (solutionB)}/{Plate count without pH regulation (solution A)}]×100

The result is shown in Table 3.

The microbicide was separately allowed to stand in a 25° C. thermostaticchamber, and the solution state was confirmed on the 19^(th) day. Theresult is shown in Table 3. (In the table, polyacrylic acid isabbreviated as PA, sodium citrate, as CA, and malic acid, as MA.)

As can be seen from Table 3, compared with Comparative Example 3 inwhich no storage stabilizer was added, Example 8 in which a storagestabilizer was added showed higher storage stability in addition to thecorrosion-inhibiting effect. Comparative Example 4 in which no corrosioninhibitor was added was large in the weight loss of test pieces.

TABLE 3 Plate Weight count Corrosion Storage loss remaining Solutioninhibitor stabilizer (g/m²) rate (%) state Example 8 PA CA, MA 0.19 0.12No Precipitation Comparative PA Nil 0.24 0.08 Rather Example 3 cloudyComparative Nil Nil 1.08 0.16 No Example 4 precipitation

INDUSTRIAL APPLICABILITY

The present invention can achieve effective sterilization whileinhibiting the corrosion of equipment piping in water treatment using amembrane separation device. Therefore, sterilization frequency can beincreased, and pH can be lowered further, to increase the sterilizationeffect.

Furthermore, in the case where a storage stabilizer is added to thewater-treating microbicide of this invention, high storage stability canbe realized while the sterilization effect and the corrosion-preventingeffect are sustained.

The present invention can be especially suitably used for the processesof seawater desalination, brackish water desalination, etc.

1. A water-treating microbicide, consisting essentially of water, aninorganic acid, a corrosion inhibitor and a carboxylic acid having 8 orless carbon atoms or alkali metal salts of the carboxylic acid, whereinthe pH of the microbicide is less than or equal to
 4. 2. Thewater-treating microbicide according to claim 1, wherein the corrosioninhibitor is polyacrylic acid.
 3. The water-treating microbicideaccording to claim 1, wherein the inorganic acid is sulfuric acid. 4.The water-treating microbicide according to claim 1, wherein theconcentration of the corrosion inhibitor is in a range of 50 ppm(weight) to 50 wt %.
 5. The water-treating microbicide according toclaim 2, wherein the molecular weight of the polyacrylic acid is 500 to10,000.
 6. The water-treating microbicide according to claim 1, whereinthe carboxylic acid is at least one selected from the group consistingof acetic acid, tartaric acid, succinic acid, citric acid and malicacid.
 7. The water-treating microbicide according to claim 1, whereinthe corrosion inhibitor is selected from the group consisting ofethylenediaminetetraacetic acid and alkali metal salts thereof, nitrousacid and alkali salts thereof, and polyepoxysuccinic acids representedby general formula (1)

wherein n is an integer of 3 or more, and x and y are, respectivelyindependently hydrogen or alkali metal.
 8. The water-treatingmicrobicide according to claim 1, wherein the inorganic acid is presentin an amount of 50 ppm–50% by weight, based on the weight of thebiocide.
 9. The water-treating microbicide according to claim 1, whereinthe corrosion inhibitor is present in an amount of 50 ppm–50% by weight,based on the weight of the biocide.
 10. The water-treating microbicideaccording to claim 4, wherein the concentration of the inorganic acid isin a range of 50 ppm (weight) to 50 wt %.
 11. A water-treatingmicrobicide consisting essentially of water, an inorganic acid in aconcentration in a range of 50 ppm (weight) to 50 wt %, a corrosioninhibitor and a carboxylic acid having 8 or less carbon atoms or alkalimetal salts of the carboxylic acid.
 12. The water-treating microbicideaccording to claim 11, wherein the concentration of the corrosioninhibitor is in a range of 50 ppm (weight) to 50 wt %.